Combining multiple CSVs pooled everything blindly; when two runs cover
the same Vin bin the user now gets a checkbox grid (row per Vin bin,
column per file, counts per cell, overlapping bins highlighted) to
decide per bin which file contributes. Opens automatically on overlap;
--pick forces it (also useful to cut bad columns from a single file),
--no-pick suppresses it for scripted use.
Also: picker/file-dialog windows force themselves to the foreground
(spawned from the GUI button, Windows opened them hidden behind the
GUI); unreadable or still-being-written (empty) CSVs are skipped with a
note instead of killing the run; fatal errors show a messagebox since
stderr is invisible when launched from the GUI.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
